1. Field of the Invention
The present invention relates to a power-supply apparatus for supplying electric power to a load that is intermittently operated by using a battery as a power supply and to a wireless communication apparatus using the power-supply apparatus.
2. Description of the Related Art
In recent years, for wireless communication apparatuses, such as mobile phone terminals, longer service life of batteries has been a challenge for higher functionality and broader applications. As secondary batteries for mobile phone terminals, at present, lithium-ion (Li-ion) batteries have been widely used. Not limited to lithium-ion batteries, when batteries are discharged, the output voltage (that is, the battery voltage) is decreased. When the battery voltage reaches the vicinity of the lower limit of the operating voltage of each unit or IC, in order to prevent malfunction, the fact that the battery has no remaining capacity is displayed, and functioning of the terminal is stopped.
A bottleneck regarding a battery voltage being low and the battery being difficult to use in a mobile phone terminal is due to a power amplifier (PA), in particular, a PA for GSM/EDGE. The reason for this is that large electric power becomes necessary to send radio waves to a base station. Since electric power=voltage×electrical current, a decrease in the battery voltage makes it difficult to obtain the maximum output specified in the communication method.
Hitherto, a power-supply circuit capable of supplying electric power to an RF power amplifier at an efficiency of approximately the same degree as in a case where a battery of a high output voltage and a regulator are used, and a wireless communication apparatus employing a time-division multiplexing method, in which a lithium-ion battery of a low output voltage is used as a power supply, have been proposed (see Japanese Unexamined Patent Application Publication No. 6-252820). In this power-supply circuit, by using a step-up DC-DC converter, a power-supply voltage supplied from a battery is pulled up to a desired voltage for a power amplifier (PA), and is stored temporarily in a capacitor, so that when transmission is to be performed, the voltage stored in the capacitor is supplied to the power amplifier. The relationship between a battery voltage Vbat and a voltage Vcharge charged in the capacitor is constantly Vcharge>Vbat.